Cord for extensible belt

ABSTRACT

An extensible belt, particularly a power transmission belt, containing a tension section of longitudinally extending individual textile cords which include synthetic thermoplastic filamentary material in their structure. The cords are simultaneously relaxed and heat-treated in the absence of applied tension and are thereby capable of an initial stretch or elongation within certain defined limits and a subsequent recovery of at least a substantial portion of the initial stretch. The cords are subsequently incorporated into the belt and the vulcanized belt is thereby made extensible within certain defined limits and capable of being stretched in order to be installed in a belt drive assembly. After the initial stretch, the belt subsequently retracts or recovers to the driving position with respect to the pulleys and retains sufficient tension to frictionally engage the pulley surfaces and perform its driving function. The self-tensioning belt of this invention eliminates the need for tensioning devices or means to adjust the drive to provide the proper tension required when conventional transmission belts are employed and is particularly useful in applications involving V-type transmission belts which are designed to operate with grooved pulleys or sheaves.

[ Aug. 19, 1975 Fix [ CORD FOR EXTENSIBLE BELT [75] Inventor: Sidney R.Fix, Lincoln, Nebr [73] Assignee: The Goodyear Tire & Rubber Company,Akron, Ohio [22] Filed: Nov. 19, 1973 [21] Appl. N0.: 417,316

Related US. Application Data [60] Continuation of Ser. No. 232,833,March 8, 1972, abandoned, which is a division of Ser. No, 26,457, March5, 1970, Pat. No. 3,666,584, which is a division of Ser. No, 739,674,June 25, 1968, Pat. No. 3,566,706.

[52] US. Cl 428/378; 28/75 R; 156/139; 428/395; 264/342 RE [51] Int. Cl.Fl6g 1/00 [58] Field of Search 161/143, 144, 170, 227, 161/231, 239;156/89,161, 139, 169, 229; 28/75 R; 118/33; 264/342 RE; 74/233 [56]References Cited UNITED STATES PATENTS 2,453,013 11/1968 Illingworth eta1 28/76 R 2,593,320 4/1952 Lewis et a1. 1. 264/342 RE 2,955,345 10/1960Howe 28/72 2,955,345 10/1960 Howe 28/75 R 3,424,833 l/l969 Mazzolini eta1 28/76 R 3,453,817 7/1969 Strub 28/76 R 3,505,164 4/1970 Oppenlander161/173 FOREIGN PATENTS OR APPLICATIONS 985,417 3/1965 United KingdomPrimary ExaminerGeorge F. Lesmes Assistant Ii.\'an1iner.lames J. BellAttorney, Agent, or Firm-F. W. Brunner; R. P. Yaist [57] ABSTRACT Anextensible belt, particularly a power transmission belt, containing atension section of longitudinally extending individual textile cordswhich include synthetic thermoplastic filamentary material in theirstructure. The cords are simultaneously relaxed and heat-treated in theabsence of applied tension and are thereby capable of an initial stretchor elongation within certain defined limits and a subsequent recovery ofat least a. substantial portion of the initial stretch. The cords aresubsequently incorporated into the belt and the vulcanized belt isthereby made extensible within certain defined limits and capable ofbeing stretched in order to be installed in a belt drive assembly. Afterthe initial stretch, the belt subsequently retracts or recovers to thedriving position with respect to the pulleys and retains sufficienttension to frictionally engage the pulley surfaces and perform itsdriving function. The self-tensioning belt of this invention eliminatesthe need for tensioning devices or means to adjust the drive to providethe proper tension required when conventional transmission belts areemployed and is particularly useful in applications involving V- typetransmission belts which are designed to operate with grooved pulleys orsheaves.

4 Claims, 6 Drawing Figures PATENTEU AUG 1 9 I975 TENSION LBS.

SET 2 OF 2 35 K (g 25 /o/ FIG 5 J 20 7 a S g .5 A LU 0 Z 4 6 8 IO l2 l4l6 la 20 ELONGATION 7 ELONGATION NOTE: BELT SAMPLE BROKE A7 FIG. 6

CORD FOR EXTENSIBLE BELT This is a continuation of application Ser. No.232,833 filed Mar. 8, 1972, now abandoned, which is a divisionalapplication of Ser. No. 26,457 filed Mar. 5, 1970, now U.S. Pat. No,3,666,584 which is a divisional application of Ser. No. 739,674 filedJune 25, 1968, now US. Pat. No. 3,566,706.

BACKGROUND OF THE INVENTION This invention generally relates toextensible belts, particularly power transmission belts, containing atension section of synthetic thermoplastic cords which have beenspecially treated prior to being incorporated into the belts to enablethe belts to be used in a unique manner. This invention also relates tothe method of making these belts and the preparation of the cords whichare contained in the tension section of the belts. Specifically, thisinvention has particular application to power transmission belts havinggenerally trapezoidal cross-sections which are designed to operate withgrooved pulleys or sheaves.

Transmission belts of flexible resilient material such as rubber ortextile fabric are normally used in drive belt assemblies to transmitpower from a driving pulley to a driven pulley by being operablypositioned in tension about and in frictional engagement with thedriving surfaces of the pulleys. These belts, like other rubber articlessuch as conveyor belts and, tires, in most instances contain a tensionsection of textile cords or square-woven fabric to provide the belt withthe necessary longitudinal reinforcement, stability, and flexibility. Ithas been found that cords of synthetic thermoplastic material such asnylon, polyester, and the like which exhibit varying degrees ofelasticity are particu larly suitable for this purpose. However, it hasalso been recognized that these synthetic thermoplastic materials alsoexhibit varying degrees of dimensional instability because of shrink andgrowth factors which are detrimental to the operation of the rubberarticle.

These factors are particularly important in regard to the functioning ofpower transmission belts. For instance, because of the tendency for thebelts to stretch or grow" after being in operation around the pulleysfor extended intervals it is necessary, in order to maintain the properoperating tension, to adjust the center distances between the drivingpulleys and the driven pulleys to compensate for the stretch of thebelt. If the pulleys are fixed so that the center distance cannot beadjusted it is necessary to provide an additional idler or take-uppulley which functions to take up the slack resulting from the naturalstretch of the belt in order to apply and continually maintain theproper belt tension. The tension of the belt is especially critical whentransmission belts of a generally trapezoidal cross-section,

commonly referred to as V-belts, are used since these belts are designedto frictionally or wedgingly engage the V-shaped grooves of the pulleysor sheaves in order to perform their driving function.

Either of the above-described belt drive assemblies are costly becauseof the movable parts and additional equipment they require. In addition,the labor costs incurred in adjusting these belts present seriouseconomic disadvantages. As those familiar with the art well know, it istherefore desirable to provide a belt drive assembly which does notrequire adjustment nor additional pulleys to maintain driving tension.

The present invention, by utilizing a unique selftensioning belt,provides such an assembly which performs satisfactorily in manyapplications. The belt of this invention'is formed endless so that itslength in its original free position is substantially less than itslength in its installed position in the belt drive assembly. It

therefore must be stretched significantly in order to beplaced in itsoperating position around the pulleys and thereafter retracts to thedriving position with respect to the pulleys but retains sufficientresidual tension to perform its driving function. This unique operatingfunction is accomplished by a special and unusual cord treatment whichwill be hereinafter fully described.

Those skilled in the art are aware that it is vital in the treatmentofheat-shrinkable cords of synthetic thermoplastic material to subjectthem to various conditions of controlled stretch, relaxation and hightemperature to increase their dimensional stability. To this end,various fabric treatments have been developed, for example, as describedin Meherg et al. US. Pat. No. 2,679,088 and Howe US. Pat. No. 2,955,345.

Accordingly, it is considered of the utmost importance to prevent anyundue stretch or elongation of the reinforcing textile materials whichare incorporated into power transmission belts since this will causeslippage of the belt as it travels around the pulleys and results in aserious loss of power. It is also extremely important to preventsignificant shrinkage of the reinforcing materials since the ensuingexcessive tension may damage the pulley bearings and cause rapid wear ofthe belt. Consequently, transmission belts such as V-belts areordinarily made essentially inextensible in dimensions which conform tothe operating length of the belt and cannot be stretched significantlyin order to be installed around the pulleys.

However, it has been found that a carefully controlled continuousprocess in which heat-shrinkable elastic or partially elastic cords,which include synthetic thermoplastic material in their structure, aresimultaneously heat-set and relaxed in the absence of applied tensionafter the rubber-to-textile bonding agent has been applied will producecords having unusual properties. These treated cords are capable of aninitial stretch or elongation within certain defined limits and asubsequent recovery of at least a substantial portion of the initialstretch, without undue elongation thereafter. Examples of the syntheticthermoplastic material are polyester, nylon, or Combinations thereof.For instance, polyester filaments or cords which are stretched orelongated up to about 15% of their original length will exhibit rapidrecovery of a large portion of the initial stretch. Those skilled in theart know that this treatment is an extraordinary departure fromestablished cord treatment procedure as described in the previouslymentioned prior art particularly in view of the well establishedpractice of stretching synthetic ther-' moplastic material subsequent tothe application of the bonding agent.

After these specially treated cords are incorporated into a rubberarticle such as a belt the belt may be elongated beyond its originalvulcanized length and will returnto a length somewhat greater than itsoriginal length but after this initial elongation it will returnsubstantially to this newly attained length upon subsequent elongation.Consequently, a unique article of manufacture is provided which is bothhighly flexible and dimensionally stable.

vWhen these specially treated cords are incorporated into the tensionsection of a power transmission belt the belt is made extensible withincertain defined limits and after being vulcanized to a predeterminedlength less than its installed length is therefore capable of beingstretched in order to be installed in the drive belt assembly. Thismaybe accomplished by either pulling or rolling the belt over the edgeof the pulleys and onto the pulley surfaces. After the initial stretchrequired for installation, the belt subsequently retracts or recovers tothe driving position with respect to the pulleys and retains sufficienttension to frictionally engage the pulley surfaces and perform itsdriving function. The extensible beltsof this invention are capable ofbeing stretched initially up to about 12% of their original free length,whereas under normal conditions, conventional transmission belts may notbe stretched much in excess of 1% without being permanently damaged. Inaddition, because of the built-in flexibility provided by thesespecially treated cords, the belts of this invention have superior shockabsorbing and resisting properties.

OBJECTS OF THE INVENTION It is a primary object of this invention toprovide an extensible belt which is both highly flexible anddimensionally stable.

It is another primary object of the present invention to provide a powertransmission belt which is capable of being stretched in order to beplaced in its operating position around the pulleys in a belt assemblybut thereafter it retracts to its driving position with respect to thepulleys and retains sufficient residual tension to perform its drivingfunction.

It is another important object of this invention to provide a belt driveassembly which does not require adjustment nor additional idler pulleysto apply or maintain driving tension.

It is still another object of this invention to provide a method formaking power transmission belts containing a tension section of cordsincluding synthetic thermoplastic material which have been speciallytreated prior to being incorporated into the belts to enable the beltsto be initially stretched around the pulleys but thereafter to retractto the driving position with respect to the pulleys and retainsufficient tension to provide the necessary driving forces.

It is a further object of this invention to provide a method forheat-treating a cord including synthetic thermoplastic filamentarymaterial in its structure whereby the heat-treated cord produced therebymay be initially elongated a substantial amount, less than the breakingpoint of the cord, but will recover a substantial portion of the initialelongation and thereafter the recovery of the cord will substantiallystabilize from any subsequent elongation to which the cord is subjected.

Other objects and advantages of this invention will become apparenthereinafter as the description thereof proceeds, the novel features,arrangements and combinations being clearly pointed out in thespecification as well as the claims thereunto appended.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a sideelevational view of the extensible belt of this invention in its freeuninstalled position.

tion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 the extensible powertransmission belt 10 of this invention formed of a basically elastomericcomposition is shown, which in this instance, has a generallytrapezoidal cross-section and is a side driving type belt referred to asa V-belt. The belt 10 after vulcanization, is normally endless and isillustrated in its original free length prior to being installed in abelt drive assembly.

FIG. 2 shows a typical belt drive assembly of the type in common usehaving a motor 11, driving pulley 12 mounted on the shaft 13 which isalso connected to the motor 11, and a driven pulley 14 mounted on theshaft 15. The belt 10 is positioned around the pulleys 12 and 14 andseated in the pulley grooves 16 and 17. The length of the belt 10 inthis installed position is greater than its original free length asillustrated in FIG. 1, and accordingly it must be stretched asubstantial amount in order to be operably positioned around the pulleys12 and 14. Since it is obvious that any reinforced belt of flexibleresilient material may be stretched at least an incremental amount, theterms stretched a substantial amount for the purposes of this inventionmean more than the amount that any of the essentially inextensibletransmission belts may be stretched or more specifically, a stretch ofabout l /2% or more.

After the belt 10 is stretched initially, it retracts to the drivingposition with respect to the pulleys 12 and 14 and retains sufficientresidual tension to perform its driving function. Therefore, as in thecase of other transmission belts when the motor 11 rotates the drivingpulley 12 by the rotation of the shaft 13, the sides of the belt 10maintain the driving tension by frictionally or wedgingly engaging theV-shaped pulley grooves 16 and 17 of the pulleys 12 and 14, and as thebelt 10 travels around the pulleys 12 and 14 it tansmits power from thedriving pulley 12 to the driven pulley 14.

This is more clearly illustrated in FIG. 3 which also shows theconstruction of the belt 10. The belt 10 is a typical V-belt formed inthe conventional manner having a body portion 18 of elastomeric materialwhich includes a compression section or cushion 19. A rubber insulatedtension section 20 is embedded in the body portion 18 above thecompression section 19. The tension section 20 includes a plurality ofat least partially elastic cords 21 which are composed at least in partof filaments of synthetic thermoplastic material such as polyester,nylon and the like or possibly combinations thereof. Preferablypolyester in the form of cable cords is selected for this purposebecause of its high dimensional stability. A rubberized fabric envelope22 for instance composed of cotton, rayon, nylon or combinationsthereof, surrounds the periphery of the belt and frictionally engagesthe edges of the V-shaped pulley groove 17 of the V-shaped pulley 14 toprovide the required driving tension. It is also possible to use amolded or raw edge V-belt for this application which contains a fabriclayer on the top of the belt but no envelope wrap.

This unique belt 10 is made extensible by a special and unusualtreatment given to the cords 21. The use of the belt 10 in manyapplications eliminates the necessity of including means for tensioningin the drive assembly which are required when conventional belts areemployed in order to apply and maintain the proper driving tension onthe belt.

The preferred procedure for treating the cords 21 which may be referredto as a heat-set relaxation process is shown in FIG. 4. Asdiagrammatically illustrated, in the manufacture of the cords 21 of thisinvention, a spool 23 of the synthetic thermoplastic cord 21 in itsbasic untreated form referred to as griege cord, is first positioned ona cord let-off apparatus (not shown). Individual cord elements 21 arethen continuously transmitted through the treatment system underessentially no tension at controlled speeds by means of the drive ortension rolls 24 and 25. Taken in order, the treatment system includesthe dipping unit 26, the heating unit 27 and the cord wind-up roll 28which includes its own drive (not shown).

In the dipping unit 26 the cord 21 travels over guide rolls 29 and iscoated and impregnated with a bath 30 of a customary rubber-to-textilebonding agent, for example, an isocyanate dip, used to effect adhesionbetween the textile cord 21 and the elastomeric body portion 18 of thebelt 10. This process takes place at approximately room temperature orfor example at about 65F. to about 85F. At any one time any specificportion of the cord 21 is in the dipping unit 26 for approximately 2seconds.

After leaving the dipping unit 26, the cord 21 is fed at a controlledrate into the heating zone of heating unit 27 by means of the tensionrolls 24 where it undergoes heat treatment. The cord 21 travels throughthe heating unit 27 and any one portion of the cord 21 is subjected totemperatures ranging from about 350F. to about 480F. for a period offrom about 60 to about 180 seconds. Those skilled in the art well knowthat the specific temperature and time depends largely upon such factorsas the size and character of the thermoplastic material being treatedand accordingly other ranges may also be appropriate. It has been found,for example, that for Dacron polyester cord having a gauge of from about.015 to about .020 inches, the preferred condition to which the cord issubjected is a temperature of about 410F. for a period of about 120seconds.

The above times and temperatures employed during the heat treatmentprocess are naturally less than those required to melt the syntheticthermoplastic cords 21 but do approximate the conditions under which thethermoplastic material will melt in order to soften the cord 21 andfacilitate its subsequent processing. One purpose of applying heat tothe cords 21, as in conventional treatment processes, is to dry andharden or set the bonding agent present on the surface of the cord 21.More importantly, the heat treatment results in a definite change in thephysical properties of the cord 21. An important advantage of the heattreatment procedure of this invention is that the drying andconditioning of the cords is accomplished at the same time instead ofrequiring an additional or subsequent heating step.

The tension rolls 25 function to pull the cord 21 through the heatingunit 27 and cooperate with tension rolls 24 to control the amount ofshrinkage the cord 21 undergoes as it is heated. This is accomplished byhaving the tension rolls 25 geared to operate at a slower speed than thetension rolls 24 so that the cord 21 is pulled out of the heating unit27 at a slower rate than the cord 21 is fed into the heating unit 27.At-the same time, thetension rolls 24 restrict the movement of the cord21 caused by the shrinkage and thereby, the cord 21 undergoes acontrolled relaxation, preferably of about 1 to about 3% of its originaluntreated length. For example, a Dacron polyester cord having a gauge offrom about .015 to about .020 inches undergoes a relaxation of about 2%when the speed of the roll 24 is 2% more than the speed of the roll 25.In this way each individual heat-shrinkable cord 21 is beingsimultaneously heated and relaxed a controlled amount in the absence ofapplied tension so that the cord 21 shrinks significantly during suchtreatment. It has been determined that the attained length of the cord21 after treatment is from about 1 to about 3% less than its originaluntreated length. It should be emphasized that no more tension is usedduring the treatment process other than that which is absolutelynecessary in order to transmit the cord 21 through the various stages ofthe treatment. The cord 21 is therefore heated while in a relaxed stateto a temperature approximating the melting point of the individual cord21 for an interval of time less than that required to melt the cord 21whereby the cord 21 will shrink to a length less than the originaluntreated length.

In the final'stage of the treatment process the cord 21 is placed on acord wind-up apparatus such as wind-up roll 28 to await further steps inthe belt making procedure.

The relative elongation characteristics of the cord 21 afterheat-treatment. is graphically illustrated in FIG. 5 in which acomparison is made with both the untreated or griege cord and cord ofthe same type which has been treated in a more conventional manner. Thetype cord tested was 2200 denier dacron polyester cable cord having agauge of about .016 to about .017 inches with 1100/2, 10 turns per inch,S twist construction.

, Average tension and elongation data are plotted in the form of curvesx, y and z. Curve x represents the plot for the untreated cord, curve yrepresents the plot for the cord treated by the methods of thisinvention, and curve 2 represents the plot for a cord which has beenstretched 6% during its cord treatment procedure. The

data indicates that the cord treated by the methods of the presentinvention has the greatest elongation at increased tension. For example,the elongation at 25 pounds pull for curve y is 14.6%, as compared to12.8% for curve x and 9% for curve z. 7

The treated cords 21 are then incorporated into the body portion 18 ofthe belt 10 to form the tension section 20 by any of the conventionalmethods well known in the art. For instance by cutting the cords 21 tothe desired length, spirally winding the cords 21 over the elastomericcushion layer 19 which has been previously placed on a building drum,and then placing another layer of elastomeric material over the cords. Arubberized fabric envelope 22 then may or may not be applied dependingupon the type of belt being formed. The belts 10 are thereaftervulcanized under heat and presment being about per cent less than theoriginal length of the raw untreated cords.

The belts A through D were formed by incorporating the specially treatedcords into a neoprene rubber formulation and a rubberized cotton fabricenvelope was sure according to well established procedures such as bymeans of a conventional curing mold, applied around the periphery ofthis structure to form The vulcanized belts of this invention arestretched the unvulcanized belt. This was accomplished by means fromabout 1 /2 to about 12% in order to be operably of a standard buildingdrum. Belt E had the same conpositioned in the drive belt assemblies.However. it struction as belts A through D except that it containedshould be realized that the higher the per cent initial no fabricenvelope on the sides or bottom of the belt. stretch of the belt thesmaller the per cent recovery and The be ts A through E thus formed werethen vulcatherefore the shorter the belt operating life because of nizedin a conventional manner. The vulcanized belts the increased possibilityof ultimate belt slippage. For contained a cushion of neoprene rubberand a tension most applications the necessary stretch is from about 2section of five (5) longitudinally extending dacron to about 5%. Example1 indicates typical test data on 15 polyester cords. Belts A through Dafter vulcanization the performance of the extensible transmission beltsof ha a length of 43 inches, a p Width of inches this invention. abottom Width of .256 inches, a thickness of .219 EXAMPLE I inches and anangle of 42. Belt E had a length of 44 inches, a top width of .417inches, a bottom width of Five power transmission belts of the V-typewere pre .234 inches, a thickness of .281 inches and an angle of paredby the method as previously described and are 36. labeled A through E.The power transmission V-type belts A through E The cords which comprisethe tension section of were installed in a suitable belt testing unithaving a cathese belts were processed in the manner described pacity of1.5 horsepower and belt operating speed of using the treatment unitsshown in FIG. 4. The type of about ,4 feet per minute. syntheticthermoplastic material selected was 2200 det A as n tall d and em ed yadjusting the nier dacron polyester cable cord having a gauge of ,016center distance between the pulleys Of the belt testing inches with 1100/2, 10 turns per inch, S twist construcunit. Belts B, C, D and E wereinstalled by being rolled tion. Polyester was chosen for thisapplication because to these pulleys and were removed by being rolled ofits relatively high dimensional stability. These cords Off these p lleysith t 'distan etw n the pulleys were oat d a d i e t d t roomtemperature remaining fixed or constant. All the belts were operwith arubber-to-textile bonding agent or adhesive cli ated at the p rtic l rhorsepower setting and for the which was composed of from about a 5 toabout a 15% number of hours Specified and then removed from the solutionby volume of isocyanate in toluene, The spetesting unit. Pertinentcomments were made as to the cific type and composition of the dip willof course decondition of the belts. The results of the tests are rependon the physical characteristics of the textile mateord d in Table A.

TABLE A Horsepower Installation Belt Transmitted Stretch* (7c) HoursCondition of Belt A 0.2 2. 3 74 Satisfactory 0.42 No reset 3 No defectsTotal 409 B .42 2. 3 292 Satisfactory 1.0 No reset i One envelope Total474 crack C l .0 4.65 339 Satisfactory Slight envelope cracking on sidewalls of belt D l .22 7.0 349 Satisfactory Some belt slippage noted dueto machine failure E 0.96 4. 1908 Satisfactory No defects The stretchpercentage noted at installation is that as the belt is seated in thepulley grooves and does not include the additional stretch required toroll the belt over the edge of the pulleys. The amount of this stretchin inches may be approximated by using the following formulation:

Stretch (inches) =Thickness of hell (inches) X 211'.

e0 rial being coated. At any one time any portion of the cord was in thedip bath for about 2 seconds.

The cords were subsequently subjected to a temperature of about 410F.for about seconds to dry and harden the bonding agent and impartshrinkage to the 65 cords while simultaneously undergoing a 2%relaxation process in the absence of applied tension which resulted inthe attained length of the cords after treat- The results noted in TableA indicate that the Belts A through D which contained the cotton fabricenvelope performed satisfactorily except for the minor defects indicatedparticularly occurring in the envelope of belts B and C. It issignificant that belt E of a molded or raw edge construction containingno fabric envelope on the sides and bottom of the belt performed verysatisfactorily when compared to belts A through D. The

envelope defects may be attributed to the considerable strain the outerfabric envelope is subjected to during the initial stretching of thebelt since the cotton fabric does not expand or extend as readily aseither the elastomeric material of the body portion or the speciallytreated elastic cords contained in the body portion of the belt. Theslippage of belt D was primarily due to equipment problems and in no wayindicates a deficiency in the performance of the belt.

The horsepower settings for belts A and B were increased withoutresetting or repositioning the belts in the testing unit. The resultsfor belt B indicate that as little as 2.3% installation stretch willcause sufficient subsequent retraction and tension to transmit up to onehorsepower. In addition. since satisfactory results are also obtainedwith an installation stretch of 7% as indicated by the results for beltD, this approximate additional stretch further indicates that each beltsize may fit a range of various size drive belt assemblies.

The unique elongation properties of the belts are illustrated in thebelt hysteresis graph shown in FIG. 6. The belt samples used inobtaining the belt hysteresis data were taken from the same productionlot of belts produced at the same time as belts A through D. it wasdetermined that the belt broke at about 16.6% elongation under a tensionof 185 pounds as illustrated Therefore, a smaller tension of 126 poundspull was applied for the purpose of obtaining the average hysteresisdata which resulted in an initial elongation from O to 12.4%,represented by curve a, and a recovery to 4.0% represented by curve a orabout 70% of the initial elongation. Curves b, b, and c, 0' illustratethat recoveries in excess of 90% are realized from the subsequentelongation of the belt indicating that the belt recovery stabilizessubstantially after the first elongation. For example, after the secondelongation to 12.6% (curve b) the belt recovers to 4.3% (curve 19') andafter the third elongation to 12.7% (curve c) the belt recovers to 4.6%(curve 0' It should be pointed out that the initial tension resulting inthe initial per cent elongation or stretch is in excess of that whichwould normally be applied in order to position the belt around thepulleys. It has been found that polyester cords will return from agreater portion of the initial elongation when smaller initial stretchesare applied for instance in the range of 10% or lower. Accordingly, itmay be expected that an initial recovery of not less than 70% will berealized when the extensible belts of this invention are utilized.

The data substantiates that the belt of this invention contains cordscapable of an initial stretch or elongation and a subsequent recovery ofa substantial portion of this stretch without undue elongationthereafter. Consequently, the belt is made extensible or stretchable andthereby capable of installation in a belt drive system by being eitherpulled or rolled over the edge of the pulleys and onto the pulleysurface but it will subsequently retract or recover to the drivingposition with respect to the pulleys and retain sufficient tension tofrictionally engage the pulley surfaces and perform its drivingfunction.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

I claim:

1. A heat-treated textile cord of the type for reinforcing powertransmission belts and comprised of filaments of snythetic thermoplasticmaterial selected from at least one of the group consisting of nylon andpolyester, said cord produced by the method consisting of heat-treatingthe cord while in a relaxed state for an interval of time of from aboutseconds to about 180 seconds and at a temperature of from about 350F toabout 480F and simultaneously relaxing the cord from about 1 to about 3%of its original length.

2. The cord as claimed in claim 1 wherein, prior to said heat treating,a liquid textile-to-rubber bonding agent is applied to the cord, andsaid agent on said cord is dried simultaneouslywith said heat treating.

3. The cord as claimed in claim 1 wherein first a textile-to-rubberbonding agent is applied to the cords and then the cords are subjectedto a temperature of from about 410F for a period of about 120 secondsand simultaneously relaxed about 2% of their original untreated length.

4. The cord as claimed in claim 1 wherein the synthetic thermoplasticmaterial is polyester.

1. A HEAT-TREATED TEXTILE CORD OF THE TYPE FOR REINFORCING POWERTRANSMISSION BELTS AND COMPRISED OF FILAMENTS OF SNYTHETIC THERMOPLASTICMATERIAL SELECTED FROM AT LEAST ONE OFF THE GROUP CONSISTING OF NYLONAND POLYESTER, SAID CORD PRODUCED BY THE METHOD CONSISTING OFHEAT-TREATING THE CORD WHILE IN A RELAXED STATE FOR AN INTERVAL OF TIMEOF FROM ABOUT 60 SECONDS TO ABOUT 180 SECONDS AND AT A TEMPERATURE OFFROM ABOUT 350*F TO ABOUT 480*F AND SIMULTANEOUSLY RELAXING THE CORDFROM ABOUT 1 TO ABOUT 3% OF ITS ORIGINAL LENGTH.
 2. The cord as claimedin claim 1 wherein, prior to said heat treating, a liquidtextile-to-rubber bonding agent is applied to the cord, and said agenton said cord is dried simultaneously with said heat treating.
 3. Thecord as claimed in claim 1 wherein first a textile-to-rubber bondingagent is applied to the cords and then the cords are subjected to atemperature of from about 410*F for a period of about 120 seconds andsimultaneously relaxed about 2% of their original untreated length. 4.The cord as claimed in claim 1 wherein the synthetic thermoplasticmaterial is polyester.